I found this link, which explains use of cams vs built in, and compares the series of symptomatic.
http://runningstitches-mkb.blogspot....-sew-603e.html
I've never sewn on a 600 series, but have on a 401, 403, and 503. The internal cam stack in a 401 is not that difficult to unstick if "frozen," reassembly means having paid attention to the order and alignment of the individual cams in the stack. (Take photos.)
There is a #23 basting cam, which I've just learned does not work in a 401, and was designed to be used in the 600+ series.